This invention relates to line protectors for communications circuits, such as telephone lines and the like.
The type of line protector with which the present invention is concerned is one that is intended primarily for protection of wire conductors and equipment connected thereto from electrical overvoltage conditions which may result from lightning, electrical power faults, and the like. Protectors for this purpose may take various forms. Frequently, they are of the type that contain an arc gap across which the overvoltage will be applied whereby an overvoltage of short duration will cause conduction across the arc gap to ground. After the short duration overvoltage condition has passed, the protector returns to its normal or nonconducting state. The arc gap may be in an arrester comprised of spaced carbon electrodes separated by air, or the arc gap may be in a sealed gas tube. The sealed gas tube arrester is essentially a cold cathode discharge tube. Gas tube arresters have a much greater useful life than arresters embodying carbon electrode air gaps.
It has been recognized that overvoltage conditions of relative long duration can cause breakdown of the ordinary type of arrester whether of the arc gap type or otherwise. With reference to gas tubes in particular, it is known to provide various fail-safe methods internally of the gas tube to protect the line in the event of failure of the gas tube due to an overvoltage condition of long duration. Arrangements of this type generally require nearly destructive conditions within the gas tube in order for the fail-safe mechanism to become operative. Such conditions can frequently result in loss of gas and consequent loss of protection before the fail-safe mechanism comes into operation. This may allow damaging overvoltage conditions to remain on the line. Furthermore, gas tubes with internal fail-safe arrangements often require an expensive construction and various compromises as to electrode materials, gas mixture and pressure, and the like, in order to allow for fail-safe operation while at the same time providing for normal short duration overvoltage protection.